Most traditional ceramic manufactured products, such as tiles and sanitaryware, are made of a ceramic body that confers shape and mechanical properties to the object; the ceramic body generally has some porosity and poor aesthetic qualities.
Said ceramic body, which is defined “green” or, alternatively, “fired”, is usually coated with a ceramic layer, called ceramic glaze; the ceramic glaze is sintered by firing, in such a way to gain suitable superficial aesthetic qualities and, in the meantime, to become a fluid-proof barrier; as a matter of fact, after firing, the ceramic glaze has usually zero porosity and is generally resistant to abrasion and to the attack of chemical agents such as acids, bases, dyes.
Glaze may be applied by dry dusting a dry mixture over the surface of the ceramic body or by applying on the surface of the ceramic body a glaze dispersed/suspended in a opportune vehicle. Traditional liquid ceramic glazes are suspensions of various powdered minerals and metal oxides that can be applied by directly dipping pieces into the glaze, pouring the glaze over the piece, spraying it onto the piece with an airbrush or similar tool, with a brush, or with any tool that will achieve the desired effect.
Liquid ceramic glazes, also called glaze slips, generally contain, dispersed in water, silica to form glass, also in form of frit (pre-fired vitreous component); in combination with a mixtures of metal oxides, usually in the form of pre-treated natural occurring minerals, such as sodium, potassium and calcium oxides which act as a flux and allow the glaze to melt at a particular temperature; alumina to stiffen the glaze and prevent it from running off the piece; ceramic pigments, such as: manganese dioxide 325 used to darken many different colors; copper carbonate used in reds, greens, and blues; cobalt oxide and cobalt carbonate used for vibrant blue pigments; and chrome oxide used for pinks, reds, and greens.
Because of the fact that most of the ingredients cited above are heavy ingredients and in order to obtain a proper coating before and after firing, it is necessary to add at least a rheology modifier into the liquid ceramic glazes that will help keeping the raw glaze batch in suspension and controlling its flow properties.
A rheology modifier is an additive which allows to opportunely regulate different parameters of the glaze such as: viscosity, pseudo-plasticity, thixotropy, binding and water retention properties, as it is well known to those skilled in the art. Typical rheological modifiers are suspending agents and thickening agents.
Suspending agents improve the stability and the flowability of the dispersion and also permit a higher percentage of suspended solids to be incorporated into the dispersion. Water-swellable clays are among the preferred suspending agents. Examples of these clays are bentonite, montmorillonite, kaolinite, hectorite, attapulgite, smectite and others. The most popular clay is standard Bentonite, which may contain small amount of iron. Another useful clay is Hectorite, which is very plastic and iron-free and belongs to the family of the smectite minerals. It is sold under various commercial names, including Bentone®, Hectabright®, Macaloid® and VeeGum®. Also synthetic smectites can be used for the same purpose.
Thickening agents, which have binding, film-forming, suspending and water retention properties, are naturally occurring or synthetically derivatized water-soluble polymer gums, such as xanthan gum, alginates, gum arabic and gum tragacanth. Modified cellulose ether gums, such as hydroxyethyl cellulose, methyl cellulose, methyl hydroxypropyl cellulose and sodium carboxymethyl cellulose, can be used. Also useful are synthetic polymers such as Carbopol® (a high molecular weight acrylic acid based polymer) and polyvinyl pyrrolidone and its copolymers. Synthetic polymers, because of their greater tendency to cause undesirable gelling of the dispersion, are less preferred than natural gums.
Even if thickening agents can be used alone in the glazes, they are usually used in combination with the suspending agents because they act synergistically to improve the rheological characteristics of the glazes.
Unfortunately, rheology modifiers, and especially water-swellable clays, are often difficult to dissolve in the thick glaze slip and, if not stirred for enough time and/or with a high shear mixer, they can create lumps or aggregates in the glaze slip. This problem can be solved by:                pre-dissolving the rheology modifier in water and leaving this solution to mature for hours (up to 24 hours),        applying a long mixing time to the glaze slip,        using very efficient mixers.        
All these solutions reduce the productivity and increase the cost and complexity of the process.
After preparation, the glaze slip is sieved in order to eliminate residual impurities and aggregates. If not completely dissolved, the lumps or aggregates of the rheology modifier can increase considerably the time required for the sieving.
Moreover, a partial dissolution of the rheology modifier can require a time-consuming correction of the viscosity of the glaze slip or, if not corrected, can cause serious glazing defects on the final products, such as leveling problems, running or crawling, which are well known to those expert in the art.
We have surprisingly found that at least one water-swellable granulated clay, at least one carboxymethyl cellulose (CMC) and, optionally, another natural gum can be suitably formulated in order to get a fast dissolving rheology modifier which can be optionally added in powder form to the glaze slip, avoiding the time consuming pre-dissolution step and at the same time reducing significantly the formation of lumps. Moreover this rheology modifier imparts very good rheological behavior to the glaze, avoiding dropping or staining problems, and has a great binding action that avoids crawling and release of the dry glaze before firing.
With the expression “water-swellable granulated clays” we mean clays in powder form which are capable of adsorbing water and have been subjected to a granulation process.
With natural gum we intend naturally occurring water-soluble polymer gum, usually polysaccharides extracted from vegetables or algae or biopolymers. Also the chemically or physically modified natural gums are included in this definition.
In the present text, by “ceramic glaze” we mean the raw material mixture that is used to produce the sintered ceramic layer used to coat ceramic bodies.
In the ceramic field, EP0368507 describes a colored ceramic glaze containing a non-granulated water-swellable smectite clay as suspending agent and a natural gum, for example carboxymethyl cellulose, as thickening agent.
As far as the Applicant knows, a mixture of a water-swellable granulated clay and carboxymethyl cellulose has not been described in the literature as rheology modifier for ceramic glaze.